This invention relates to a positioning control system for use in positioning control of power devices such as an electric motor and a hydraulic cylinder and, more particularly, to a positioning control system performing a positioning control in an open loop and, still more particularly, to a positioning control system performing a positioning control in which a lead angle is compensated in accordance with a sliding compensation amount determined on the basis of an amount of sliding during braking.
A prior art example of a device performing a positioning control in an open loop by employing a braking system such as a brake is shown in FIG. 9. A position of an object of control is detected by a position detector 1 and the position detection data is compared with a set value of a target position by a comparator 2 to provide a signal for controlling an actuator 3 such as a brake in accordance with a target position. In this case, speed of the object is detected by a speed detector 4, sliding compensation amount data is computed by a sliding compensation amount operation circuit 5 in accordance with the detected speed and the position detection data or target position data is corrected in accordance with this sliding compensation amount data. More specifically, the sliding compensation amount operation circuit 5 can produce a function of an expected value of a brake sliding amount relative to the speed (since this is a prepared value, this value is hereinafter referred to as "preparatory learning value") by storing or calculating such value and provide a preparatory learning value of brake sliding amount in this function as sliding compensation amount data s in accordance with the detected speed v. Current position data of the object is advanced in appearance by this sliding compensation amount s by adding the sliding compensation amount data s to position data x detected by the position detector 1 (or subtracting this data s from the target position data). Brake is thereby applied earlily by this sliding compensation amount data s and if, ieally, actual brake sliding amount is equal to the sliding compensation amount s, the object can be stopped accurately at the target position.
Such preparatory learning function however is generally not sufficient for securing a sufficient accuracy and, for securing such accuracy, the applicant has already proposed an open loop control system having a learning function which may be called a review function. According to this review function, an error between the actual stop position in the preceding positioning operation and the target position is stored and a further compensation operation is made by using this error as a review value in next positioning control.
An example of a positioning control device having such preparatory learning and review functions is disclosed in U.S. Pat. No. 4,651,073.
In the above described positioning control device, it is possible to select one of functions of preparatory learning value manually in accordance with operation condition of the machine or the like. Once a function has been selected, however, the selected function only is always used and it cannot be changed automatically. Such function of preparatory learning value generally has a function of second order or multiple order as shown in FIG. 10.
FIG. 10 shows an example of a selected preparatory learning function by a solid curve and an example of a function of actual speed-brake sliding amount by a chain and dot line. Such relatively large difference between the manually selected preparatory learning function curve and the actual speed-brake sliding amount curve frequently occurs. If the speed does not vary greatly, such difference in the expected value can be compensated relatively easily by the above described review function. If, however, the speed has dropped sharply due to abrupt change in the load condition in the mechanical system in the object or other reason, the difference can no longer be compensated by the above described review function.
Let us assume a simple example for convenience of explanation. At speed V1, S1 is used as the preparatory learning value (sliding compensation amount) and actual sliding amount is Sa so that data corresponding to difference d between S1 and Sa is the amount of compensation as a review value. If the speed at which brake is applied in each positioning is substantially in the vicinity of the same V1, the compensation by the review value d brings about a good result. Even if the speed varies, the review value can follow a gradual change in the speed so that no serious problem occurs. If, however, the speed drops sharply from V1 to V2, the preparatory learning value is S2 and the review value is d which is the same as in the preceding time. Accordingly, compensation is made on the assumption that Sc while is sum of S2 and d is the sliding amount while actual sliding amount is only Sb and, as a result, a relatively large error is produced. One reason for such large error is difference in inclination between the preparatory learning function curve and the actual speed-brake sliding amount curve. The fact that these curves are second order or multiple order functions further increases such error. In short, the higher the speed, the larger is the gap between the two curves.
In the open loop positioning control, whether or not an acutal stop position coincides with a target position within a predetermined permissible positive and negative error range (this permissible error range will hereinafter be referred to as "coincidence width") is checked and, if the actual stop position is within the predetermined coincidence width concerning the target position, it is judged that correct positioning at the target position has been achieved. If the actual stop position is not within the coincidence width concerning the target position, positioning is inaccurate so that the positioning control is once stopped and a proper step such as retrial of positioning must be taken. If the above described preparatory learning control and review control are performed, such inaccuracy in positioning will seldom take place. Even in such control, a positioning acutator (e.g., fluid pressure cylinder such as a pneumatic cylinder) may suddenly be actuated by an accidental external force due to some factor, e.g., vibration characteristics of the mechanical system, immediately after generation of a brake signal and immediately before stopping with a result that the actual stop position comes ouside of the coincidence width. Particularly in the case of a sequential positioning control in which positioning is made sequentially with respect to target positions of plural steps, deviation from the coincidence width not only brings about inconvenience due to inaccurate positioning in a particular step in which such deviation has occurred but also has adverse effects on positioning in next step so that the sequence control must be stopped and the machine must be stopped. Particularly, in a process in which work is done by a tact movement of positioning control systems such as an automobile manufacturing line, deviation in one positioning obliges stoppage of the entire tact so that it reduces efficiency of the work.